1. Field of the Invention
The present invention relates to an economical process for the production of aluminum-lithium alloys. More particularly, this invention relates to the manufacturing process of aluminum-lithium alloys by use of an aluminum-sealed lithium ingot which is made by cold extrusion of pure lithium into an aluminum container. This aluminum-sealed lithium ingot prevents highly reactive lithium from oxidation in atmosphere and thus enables an atmospheric melting of aluminum-lithium alloys.
2. Description of the Prior Art
Lithium has a specific gravity of 0.534, which is the lowest value of all metals and alloys. Pure solid lithium is so ductile that it can be easily cut and worked at room temperature. Lithium is an extremely reactive material, so that it is easily oxidized when exposed to atmosphere. This reactive property gives a difficulty in control of this material in an atmosphere.
The addition of lithium to aluminum gives promising effect of a considerable improvement of specific strength and specific modulus making this alloy system extremely attractive to the aerospace industry. It has been known that the addition of each wt % lithium results in a 3% reduction in density and a 6% increase in the Young's modulus. One alloy, recently registered as 2090, which has been introduced as a replacement for 7075-T6 products achieves a potential weight saving of 8 to 10%.
However, there are many difficulties in the fabrication of aluminum-lithium alloys. In the production of aluminum base alloys, it is common to practice melting and casting in an open atmosphere and add the alloying elements in the form of a master alloy or pure metals. Because of the high reactivity of lithium, however, aluminum-lithium alloys are difficult to melt and cast and also possess poor mechanical properties when processed by conventional ingot making method.
In conventional process, the loss of lithium due to its rapid oxidation rate is undesirably high (over 20%). This, therefore, makes it difficult to control the composition of these alloys and the process is not economical. At present, thus, it is generally known that the manufacturing process of aluminum-lithium alloys is entirely performed in an inert atmosphere.
As a method for producing aluminum-lithium alloys, it was proposed in U.S. Pat. No. 4,556,535 to fabricate these alloys by a process which comprises introducing molten lithium beneath the surface of an agitated source of molten aluminum while bubbling a mixture of argon and chlorine gases through the molten metal mixture and continuously casting an aluminum-lithium alloy ingot while monitoring the ingot casting rate.
The manufacturing process like this, however, requires high cost in planting and processing because an inert atmosphere must be kept throughout the process. Sophisticated technical knowhow and some complicated techniques are also required to operate the system. In this regard, a fabricating method of high quality aluminum-lithium alloys at low cost is necessitated.